Population biology of humpback dolphins in Richards bay, South Africa

Abstract

During the last 19 years a minimum of 72 humpback dolphins Sousa chinensis were retrieved from the Richards Bay shark nets. Recently developed acoustic warning devices led to the introduction of these devices (pingers) into the shark nets at Richards Bay in an attempt to reduce the number of humpback dolphins captured in the nets. With the KwaZulu-Natal population set at approximately 200 animals this step led to the conception of the Richards Bay Humpback Dolphin Project. The present study was designed to obtain data that would reflect the state of the humpback dolphin population frequenting the Richards Bay area. The study was conducted during April to October of 1998 at Richards Bay (28° 48' S, 032° 06' E), on the eastern coast of KwaZulu-Natal. Humpback dolphins frequented the Richards Bay area throughout the study period with the average group size being 8.72 (± 5.13). Data suggests that the Richards Bay area may be a suitable nursery are~ since "females" with calves in close attendance, were frequently sighted in the study area. Humpback dolphins were seen most frequently in the southern and harbour mouth region of the study area and these areas most likely represent preferred areas. Collection of biotic and abiotic data throughout the study are~ such as water visibility, water temperature and ocean depth were used to correlate any noticeable environmental differences in area utilisation of humpback dolphin groups found in the study area. Only water depth seemed to limit the distribution of this species. No preference for turbid waters were evident during the study. Little or no preference for certain water temperature regimes were evident. Analysis of capture data of humpback dolphins in the Richards Bay shark nets revealed an average capture rate of 3.79 (S.D. ± 3.08) dolphins per year during the 19- year period of installation. The number of dolphins captured in relation to the total length of nets in the particular installation do not seem to be correlated. Peaks in captures existed during the winter months (June to August). Humpback dolphin follows within the harbour and harbour mouth occurred primarily from June to August, the period which coincided with the dolphin captures of 1998 in the Richards Bay installation. Differences in seasonal utilisation and movement of humpback dolphins may lead to an increase in the number of dolphins caught during the winter months within the Richards Bay shark net installation. The number of newly identified individuals did not decrease with time, suggesting that the population sampled at Richards Bay may appear to be an open population. Using all identified individuals sighted during this study, a population of 213 (± 42.72 at the 95% confidence level) individuals was calculated. The continual discovery of new individuals in conjunction with some individuals exhibiting long distance movements along the coast, indicate that a large part of the sampled population does not constitute dolphins actually "resident" within the study area. As the majority of the identified individuals (63 % ) were only photographed once within the study area, unfamiliarity may be one of the main reasons contributing to the high humpback dolphin capture rate in the Richards Bay installation. Unfortunately, social structure estimates of humpback dolphins were severely hampered by irregular movements of dolphin groups. Active acoustic deterrent devices (pingers) were introduced during June 1998 into the Richards Bay shark net installation in an effort to reduce the rate of capture of humpback dolphins in the shark nets. Humpback dolphins frequented the netted areas with and without pingers in place and even though three dolphins were caught in the shark nets, no final conclusion could be made about to the efficacy of the pingers, as all three captures took place during non-active pinger (dummies) periods. Continued investigation into the efficacy of the pingers is therefore needed. The observed site fidelity and home ranges indicated that the Richards Bay area may be for some individuals a "core area" or "preferred area" while others individuals just pass through. Humpback dolphins studied in the Richards Bay area displayed differences in site fidelity and had distinct levels of residency to the study area, affected by various unknown factors. In conclusion, the continuous monitoring of the KZN humpback dolphin population is imperative, to ascertain any change in the current status, as population depletion in conjunction with mass habitat destruction, usually go un-noticed and unchallenged.